The inhomogeneous nature of the Earth's mass distribution, particularly the equatorial bulge, perturbs the orbits of all Earth satellites. Three perturbations are most prominent: (I) the orbit plane rotates around the earth in inertial space ("orbit plane precession"); (2) the orbit's line of apsides (i.e., the major axis of an elliptical orbit) rotates within the orbit plane ("apsides precession"); and (3) the orbit's radius vector oscillates about the mean orbit, causing the orbit radius to decrease at high latitudes and increase near the equator. For low Earth orbit ("LEO") spacecraft, even in elliptical orbits, the third perturbation is small and can usually be ignored, but the first two effects are quite pronounced.
Some of the fundamentals of orbital mechanics, and of Earth orbital mechanics in particular, and astronomical systems of measurement are set forth in the prior art, which includes U.S. Pat. Nos. 3,836,969 to Bond et al., 4,084,772 to Muhlfelder, 4,776,540 to Westerlund, and the references therein cited, and in A. Roy, "Orbital Motion", 3d ed., Adam Hilger, Philadelphia (1988) and J. Wertz ed., "Spacecraft Attitude Determination and Control", Kluwer Academic Publishers, Boston (1978). In those references, orbit plane precession and apsides precession are treated as effects to be eliminated by geostationary satellite station-keeping systems. In contrast as described below, the present invention exploits those effects to produce desired orbital geometries.
The successful flight in April 1990 of the first PEGASUS.TM. launch vehicle has verified that the price per kilogram of mass to LEO can be maintained even when the total mass of the satellite system being launched is low. The capabilities of such vehicles provide many new options to small-satellite designers that have not been considered because they have never before been available. Selections of orbits, launch windows, and deployment strategies have never been available for small satellites because they have usually been carried as secondary payloads on large launch vehicles. The advent of PEGASUS.TM.-class launch vehicles makes practical a distributed LEO network of multiple satellites in multiple orbit planes providing global network coverage.
Although some aspects of global satellite networks have been considered since the beginning of the space era, until now the cost of such networks has been prohibitive. By using a single PEGASUS.TM.-class launch vehicle for each satellite or for each orbit plane, the aggregate cost of a LEO satellite network can be of the same order of magnitude as that of conventional global geostationary satellite networks.
At the same time, technology has advanced to the point that communications and scientific payloads having commercially significant capabilities can be incorporated into spacecraft having masses as low as 10 to 20 Kg (20 to 40 lbs); such spacecraft are referred to as "micro-satellites". Although micro-satellites are much smaller than typical communication satellites, which can have masses of hundreds or thousands of kilograms, it will be appreciated that a global LEO network's value (e.g., its capacity, in the communications sense) is derived from the aggregate of the satellites, not from any single member of the satellite network. Even now, as in the past, this important point is usually missed during consideration of microsatellite designs that, despite their "cuteness," are not physically impressive.
To date, no global LEO satellite network has been put in place, although aspects of such networks have been considered, and such networks are now being proposed by a variety of commercial entities. One such proposal has recently been made by Starsys, Inc. in a public filing at the Federal Communications Commission. General aspects of satellite network configurations are described in Adams et al., "Circular Polar Constellations Providing Single or Multiple Coverage above a Specified Latitude", J. Astronautical Sci. vol. 35, no. 2, pp. 155f-192 (April-June 1987).
The usual approach taken in studying global LEO satellite networks is to assume that all satellites from a single launcher are placed in a single orbit plane; solutions are then found to distribute the satellites around the orbit plane, usually in a uniformly spaced manner. One such network is described in U.S. patent application Ser. No. 07/485,655 filed Feb. 27, 1990, for a "Mobile Satellite System" by A. Elias et al. and assigned to the assignee of the present application.
Through the present invention, however, an entire global network of micro-satellites could be deployed using a single PEGASUS.TM.-class launch vehicle. Indeed, a wide variety of network configurations can be implemented according to the needs of network customers and various trade-off parameters as discussed below.
One such network concept, called "geobeacon," involves the use of an ultra-precise radio location technique to determine the distance between two locations separated by a large (approximately continental) distance to an accuracy of a few centimeters. Such accuracy is comparable to that achieved with Very Long Baseline Interferometry, which is a current method used to monitor motion of the Earth's tectonic plates, and several microwave uplinks to each satellite and a single downlink would be employed to carry out the (three-dimensional) distance measurement. Nearly continuous coverage is desirable for the geobeacon concept. Since seismic equipment is typically located at the sites (estimated to be between 100 and 1000 in number) for which the distance measurements would be made, it would be possible to relay seismic data from those remote sites using the same satellite network. As described in more detail below, a network suitable for the geobeacon concept could probably be provided by 16 micro-satellites deployed by one PEGASUS.TM.-class launch vehicle.
Another need that could be met through the present invention is that for a network for monitoring the Earth's dynamic magnetic field. Such measurements have been carried out intermittently by dozens of spacecraft since the late 1950's, but there is a current need for continuous monitoring by a global network in a correlative fashion. By use of the present invention, a single launch vehicle could deploy the network, which might include more than 4 satellites per orbit plane in more than 4 planes and provide continuous data over many years, e.g., for at least one solar cycle (22 years).
As for the Mobile Satellite System ("MSS") described in the above-identified U.S. patent application, which is expressly incorporated into the present application by reference, use of the present invention could provide a network having about 25-33% of the communications capacity of the MSS at an estimated cost of less than about $20 million. Each satellite could support one continuous 8-10 Watt downlink and a second 10 Watt downlink for short durations, providing a commercially significant capability for several types of user. The receiver in each satellite and communication data rates and modulation formats could be those described in the MSS patent application.